Many aquatic animals are highly sensitive to specific xenobiotics. Such organisms are used as models to identify those physiological processes most sensitive to environmental pollutants. Since the exposed location and functional importance of cell membranes make them particularly susceptible to the toxic effects of foreign chemicals, we have focused primarily on the interactions of xenobiotics with membrane transport systems, particularly transport in epithelia where the coupled function of apical and basal-lateral membranes provided the basis of physiological control mechanisms. We have used kidney and gill since (1) function of these organs depends in large part on membrane transport, (2) they both play important roles in determining the rate at which many foreign compounds are excreted from the body, and (3) each is vital to the overall homeostasis of the organism.